According to the study carried out by the inventors of the present invention, with regard to a DC-DC converter, the technology as follows has been known.
In recent years, along with the increasing demands for low voltage and high current for a CPU (Central Processor Unit) and a MPU (Micro Processor Unit) that are used in a personal computer, a server and the like and downsizing of passive components such as a choke coil and an input/output capacitor, higher efficiency and higher frequency are required for a power supply system that supplies power to the CPU and the MPU.
A non-insulated DC-DC converter for such a system comprises a high side switch and a low side switch, and power MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) are used for each of the switches.
However, in the non-insulated DC-DC converter, a self turn-on phenomenon sometimes occurs in which turn-on of the high side MOSFET causes erroneous turn-on of the low side MOSFET. This self turn-on phenomenon gives rise to a problem of significant reduction in power efficiency.
For prevention of the self turn-on, it is effective to apply a sufficient negative voltage between a gate and a source of the low side MOSFET in a period when the low side MOSFET is in an OFF state. More specifically, by applying a sufficient negative voltage between the gate and the source of the low side MOSFET, the gate-source voltage of the low side MOSFET does not reach a gate threshold voltage of the low side MOSFET even when the increase in the gate-source voltage that induces the self turn-on occurs.
However, a DC voltage source whose negative terminal is connected to a source terminal of the low side MOSFET is used in general for a power source for a gate driver of a non-insulated DC-DC converter having a small power capacity, and a negative voltage cannot be applied between the gate and the source of the low side MOSFET.
As a solution for such a problem, for example, Japanese Patent Application Laid-Open Publication No. 8-149796 (Patent document 1) discloses means for applying a negative voltage between a gate and a source of a voltage controlled device by a gate driver to which a capacitor is added.